1. Field of the Invention
The present invention relates generally to a viscous fluid coupling device. In particular, the present invention is concerned with an electromagnetic control assembly of a viscous fluid fan clutch for a vehicle.
2. Statement of the Related Art
A viscous fluid clutch assembly for driving and rotating a vehicle cooling fan is well-known. A multi-bladed fan is removably secured to a clutch body of the assembly. The fan and clutch assembly are installed between an accessory pulley of a vehicle engine (typically the water pump pulley) and a radiator. The clutch assembly drives the fan at high speeds close to input speed when cooling is required and permits the fan to rotate at low speeds when cooling is not required.
Typically, a clutch assembly utilizes a control device to vary the speed of the fan. When cooling is required, the control device causes the torque from an input shaft to be transferred to the attached fan. When cooling is not required, the input torque is not transferred to the fan, thereby reducing the load on an engine and improving efficiency.
In many clutch assemblies, a helically-wound bimetallic coil is utilized as a control device for changing the speed of a fan. Ambient air temperature causes winding and unwinding of the bimetallic coil so as to rotate a valve plate and control flow of fluid inside the clutch assembly. When fluid gates are opened, fluid travels to a shear zone so that fluid shear transmits input torque to the fan. When fluid gates are closed, the majority of fluid is pumped out of the shear zone and the fan speed is reduced. A representative example of this type of fan clutch can be found in U.S. Pat. No. 4,741,421.
The art continues to seek improvements. To improve efficiencies, it is desirable to monitor the temperature of an engine coolant fluid. When the temperature of a coolant fluid reaches a predetermined level, the engine cooling fan should be actuated. It is desirable that a viscous fluid clutch assembly include a control device for controlling the speed of a fan based on the temperature of the engine coolant fluid and other strategically-monitored temperatures.